Good afternoon.

One of my players came today to me with a story that he had to reconnect in order to regain access to his backpack. After he teleported from base A to base B with Nteleport plugin, he could not access his backpack. 

We asked him to try the backpack UI command as well as the normal backpack command. Even his keybind failed. 
There are no logs of an error to be found, so I'm wild guessing here what could have gone wrong. 

I have no reason to believe that teleporting alone could cause the backpack to stop being openable. Therefore, it is most likely that something else happened which caused this, around the same time.

Do you have any capability in your server that dynamically changes permissions, or that dynamically prohibits Backpacks from being opened in particular circumstances (such as in particular zones)?

If it is none of these things, the closest issue in the past that this resembles, is a problem I fixed last year where players would be unable to open their backpack if they died while offline. When that bug manifested, no errors would be displayed anywhere. Given that you similarly had no errors in this case, it might be the same code path where the issue is happening, though likely due to an undiscovered root cause. There aren't very many code paths where no error would be printed, so we can take a look to narrow down the possible circumstances that could cause this, to get a better idea of how to reproduce.

There is also the possibility that it was a client side issue. I suggest this because there have been no other reports of this, and because clients may in some cases automatically close containers when opened, if particular conditions are not met. If this is the cause, it is likely just a rare client bug.

First of all, thank you for your reply. Not many provide such a large amount of information. 

I do think indeed that this might have been caused on his own side. as what you mention indeed, there is no one else having similar issues. I do hear sometimes people losing their rights to use the backpack and that, once they relogging that they have access again. 

about your question depending on zones or/and dynamical permissions, we do have a few plugins running active that blocks the use of backpacks when inside, however. the user that had the problem teleported from one base to another, so he/she was not inside such zone. 

The last few days, my server is tanking massively in Server FPS. Could it be somehow related to that? I doubt it, but it's worth taking any information possible ofcourse.